There are examples of such a request on the page you linked. You should probably use the POST method (last example) rather than a more involved SOAP request method. You can build the query string manually or using QUrlQuery (5.3) or QUrl (4.8). Build a QNetworkRequest pointing at the web service URL (you can find these in the WSDL linked from that page), set the Content-Type header as per the example, and call QNetworkManager:ost().

This is a generic HTTP POST process and there are more than a few examples in these forums and the Qt Project forums.